Multi-layered molded articles

ABSTRACT

A multi-layered article molded according to the present invention has in its cross-section a triple-layered structure consisting of two surface layers and a core layer interposed between said surface layers, said triple-layered structure being injection molded in a single shot from two kinds of thermoplastic synthetic resins to be formed into the surface and core layers, respectively, said core layer having a thickness less than one half of the entire thickness of said article.

The present invention relates to a multi-layered article made ofthermoplastic synthetic resins which has a triple-layered cross-sectionwith the core layer thereof having a thickness less than one half of theentire thickness of the article and which is superior in at least one ofvarious properties such as gas permeability, high-impact property,rigidity, appearance or the like. The present invention also relates toa process and apparatus for injection molding the above-mentionedmulti-layered article.

Multi-layered articles made of two or more kinds of synthetic resinshave been recently developed and accepted for many applications such asvessels for food. When articles are required to have a plurality ofproperties such as rigidity and barrier ability or strength and chemicalresistance, it is difficult to satisfy such requirements by only asingle kind of synthetic resin. Otherwise, remarkably expensivesynthetic resins must be used to obtain articles having the desiredproperties. However, the multi-layered articles can satisfy therequirements as mentioned above by using two or more kinds ofinexpensive resin materials each of which has at least one of thedesired properties.

Vessels having multi-layered structures have been produced in many casesby blow molding preforms which had previously extrusion molded. However,such products have many disadvantages. For example, they tend to havediscontinuous weakened portions or excessive materials to be trimmed.

An injection molding process or the combination of a blow moldingprocess therewith is valuable since it does not have the disadvantagespresent in an extrusion molding process. In addition, such a moldingprocess has many advantages in that it is possible to obtain highlyaccurate products due to superior duplicability and due to injectionmold preforms which are very suitable for forming them in the blowmolding operation. If the aforementioned multi-layered articles wereobtainable by the injection molding process, one would produceeconomically the articles which are extremely preferred as vessels.

The injection molding process for producing the multi-layere articlescan be classified into the following methods:

(1) A first method comprising the steps of injecting two or more kindsof synthetic resin into a mold cavity under a laminar flow condition andholding such laminar flow condition in the mold cavity until it isfilled with the synthetic resins. For example, see U.S. Pat. No.3,339,240.

(2) A second method comprising the steps of injecting first of all onlythe kind of synthetic resin to be formed into the surface layers intothe mold cavity in the desired amount and subsequently injecting anotherkind of synthetic resin to be formed into a core layer until the moldcavity is filled with these synthetic resins. For example, see U.S. Pat.No. 2,996,764 and DT-OL No. 2,308,571.

(3) A third method comprising the steps of injecting first of all onlyone kind of synthetic resin to be formed into the surface layers intothe mold cavity in the desired amount and subsequently injecting anotherkind of foamable synthetic resin to be formed into a core layer as wellas the synthetic resin for the surface layers until the mold cavity isfilled therewith to form a foamed structure. For example, see Swiss Pat.No. 528,359.

(4) A fourth method comprising the steps of placing a preform which hasbeen previously injection molded in a cavity which resembles in shape apreform but of the dimension greater than it and subsequently injectingfurther the material around the preform in the mold cavity. For example,see Japanese Patent Publication No. 29980/1971.

The first method is used in a case where a multi-layered article hasouter and inner surface layers made of different kinds of syntheticresins or where a kind of synthetic resin for forming an outer layer hasits viscosity equal to that of another kind of synthetic resin forforming an inner layer with the layers having the same thickness.However, when the viscosity and thickness of the outer layer aredifferent from that of the inner layer, the laminar flows of thesynthetic resins moving in the mold cavity tend to unbalance at theirleading portions so that they will be mingled with each other resultingin a molded article with no desired layered structure. Moreover, whenthe first method is used to produce a molded article having the outerand inner surface layers of the same material and the core layer ofdifferent material, the apparatus for carrying out the first method isextremely complicated and expensive due to the needs of a nozzle havingtwo or more manifolds and the like.

The second method is called "Sandwich injection molding" wherein amachine used for carrying out this method is relatively simple instructure. The second method can use synthetic resins of differentviscosities, but the molded products obtained having thick core layerswhich cannot be optionally varied.

The third method is used to produce certain foamed structures having afoamed core layer as well as unfoamed surface layers. This method doesnot take into consideration two kinds of synthetic resins injectedsimultaneously into the mold cavity.

The fourth method is preferred for producing a multi-layered articlehaving surface and core layers of relatively great thickness. However,in this method it is hard to mold an article having layers thinner than1 mm since it is required to use a very large injecting pressure duringthe injecting operation. Moreover, the fourth method is economicallydisadvantageous since it is necessary to use a number of molds as wellas many molding operations.

We have found the fact that multi-layered articles can be injectionmolded of thermoplastic synthetic resins with the core layer thereofinterposed between the surface layers and having the desired thicknessand position by injection first a first kind of synthetic resin forforming the surface layers into a mold cavity and subsequently a secondkind of synthetic resin for forming the core layer as well as anotherbody of the first synthetic resin at the same time or sequentially intothe mold cavity, and maintaining within specific limitations the amountof the first synthetic resin firstly injected, the ratio of flow ratebetween the first and second synthetic resins simultaneously injectedand the ratio of melt viscosity between the two synthetic resins.

The present invention provides multi-layered molded articles made of twokinds of thermoplastic synthetic resins for forming surface and corelayers and having the shape of vessel, closed-end parison, disk or thelike, each of which has the triple-layered cross-sectional structurewith the core layer thereof interposed between the surface layers of thearticle, the core layer having its thickness less than one half of theentire thickness of the article.

In such a triple-layered structure, the surface layers are layersdefining each side face for a plate-like article, and layers definingouter and inner wall for a cylindrical or vessel-like article. In anycase, the core layer means a thin layer extending uniformly between thesurface layers over the entire article. As previously described, thetriple-layered article as above-mentioned provides various useful moldedarticles having the desired properties. For example, the presentinvention provides vessels having a stable gas barrier and low watervapor permeability by using gas barrier synthetic resin as the surfacelayers and little water vapor permeable synthetic resin as the corelayer.

The technique for injection molding two kinds of synthetic resin toproduce multi-layered articles is classified into the followingprocesses:

a process characterized by a substantially simultaneous injection of thesynthetic resins including the steps of injecting the synthetic resinaccumulated in two injecting cylinders by a single shot, injecting thesynthetic resins accumulated in a single cylinder without mingling witheach other by a single shot, or injecting firstly only one kind ofsynthetic resin and subsequently injecting another kind of syntheticresin before the firstly injected material is cooled to prevent anothermaterial from flowing in; and

a process characterized by the steps of cooling completely a preformwhich has been injection molded by a first shot and thereafter placingthe preform within another mold cavity having a similar shape butgreater dimension to form a space around the preform placed in the moldcavity, the material being injected into said space around the preformby a second shot.

The present invention belongs to the former type of process which ispreferred in comparison with the latter process wherein a plurality ofexpensive molds is required with the molding steps increased, and themolded articles are generally limited to having the thickness of eachlayer thicker than 1 mm.

The fact that the thickness of the core layer is less than one half ofthe entire thickness of the article means that the core layer thicknessis less than the total thickness of the surface layers. This means thatthe core layer having gas barrier properties can be economicallyproduced using a gas barrier resin which is expensive.

Moreover, the present invention provides particularly preferred moldedarticles having thicker hardening layers which are formed in the surfacelayers at the portion thereof contacting the inner walls of the moldcavity since the core layer can be formed with the thickness thereofless than one half of the value which is subtracted by the thickness ofthe hardening layers from the entire article thickness.

Stated in other words the "hardening layer" is the resin layer which iscooled and sets and is located adjacent to the inner wall of the moldimmediately after the mold cavity has been filled with the desiredamount of resin.

The present invention further provides extremely useful multi-layeredarticles which have their cross-sectional structure represented by

    a+b<1/2(a+b+c)

where a is the thickness of the core layer, b is the thickness of thethinner surface layer and c is the thickness of the thicker surfacelayer.

In multi-layered articles injection molded according to the prior art,the core layer must be increased in thickness when it is desired toposition the core layer as near the surface layer as possible. However,in the multi-layered articles injection molded according to the presentinvention which have their cross-sectional structure represented bya+b<1/2(a+b+c), the core layer can be positioned near the surface layerwithout increasing the thickness of the core layer.

Thus, the present invention provides multi-layered molded articleshaving many practically useful advantages as described hereinafter.

In general, the core layer is formed by expensive synthetic resinmaterials having particular properties such as barrier ability, highstrength and the like. It is therefore apparent that the thinner thecore layer, the more economically the article is produced.

In cylindrical or similarly shaped vessels, they can be made of a lesseramount of the synthetic resin by using the core layer positioned in alocation near the inner wall of the vessel rather than the centralportion of the wall thickness when the core layer is very thin incomparison with the entire wall thickness of the vessel. This is aremarkable advantage in economy when a large number of vessels areproduced.

In a cylindrical multi-layered article used as a vessel containingpressurized fluid such as a carbonated beverage, sufficient strengthwould be obtained in spite of the thinner wall thickness if the corelayer of high strength synthetic resin is formed near the inner wall ofthe vessel which is subjected to the largest stress.

When a localized force may be applied to the outer wall of the vessel,the core layer of high strength synthetic resin is formed near the outerwall of the vessel to prevent any crack in the outer wall from reachingthe inner wall of the vessel.

In a triple-layered molded article having an outer transparent surfacelayer and a colored core layer, it is well known that in appearance itbecomes deeper in color as the outer surface layer increases inthickness. According to the present invention, the same advantage may beobtained by forming the core layer near the inner wall of the articlewithout increasing the wall thickness thereof.

It is apparent from the foregoing that the present invention enables thecore layer to be formed in the wall thickness of an article at thedesired position between the outer and inner walls. The presentinvention may be preferably applied to articles such as vessels and thelike in which the core layer is used with an extremely thin thickness,for example, less than one fourth of the entire wall thickness.

The thermoplastic synthetic resins used for forming the surface and corelayers according to the present invention include any thermoplasticsynthetic resin which is commercially available. The synthetic resin forthe core layer is preferably selected from the group consisting of vinylchloride-vinylidene chloride copolymer, saponified ethylene-vinylacetate copolymer, nylon acrylonitrile-styrene copolymer containing amajor amount of acrylonitrile and blends thereof. These resins aresuperior in gas barrier ability.

The high strength synthetic resin may be selected from the groupconsisting of acrylonitrile-butadiene-styrene copolymer, polyethyleneterephthalate, the above barrier resin modified by glass polyacetal andpolycarbonate and the like. Moreover, any additive such as colorant,etc. may be used together with each of the above synthetic resins.

It is to be understood that the present invention also includes the useof only a single kind of synthetic resin. In such a case, the surfacelayers and/or the core layer may be formed by the same synthetic resincontaining any suitable additive.

When the different kinds of synthetic resins are not miscible with eachother, any adhesive resin may be blended with one or both of thedifferent kinds of synthetic resins so that the connection between thesurface and core layers may be prevented from debonding. This ispreferred particularly when the surface layers are formed by apolyolefin and the core layer is formed by a blend of saponifiedethylene-vinyl acetate copolymer and polyolefin, the weight ratio ofsaid blend being 10-90:90-10.

In order to accomplish the same purpose, the core layer may be formed bya blend consisting of saponified ethylene-vinyl acetate copolymer,polyolefin and thermoplastic polymer in which the carbonyl grouppresents at a concentration of 120-1400 meq per 100 g of said polymer onits backbone chain or side chain, said polymer being present in therange of 0.5 to 15 parts by weight per 100 parts by weight of the totalamount of said polyolefin and saponified ethylene-vinyl acetatecopolymer. The ethylene-vinyl acetate copolymer described herein definesa copolymer containing ethylene in an amount of 25 to 50% by mol andhaving a saponification degree of more than 96%. The thermoplasticpolymer has, on its backbone chain or side chain, carbonyl groups basedon liberated carboxylic acid, carboxylates, carboxylate amide,carboxylic anhydride, carbonate, urethane and urea at a concentration of120-1400 meq per 100 g of said polymer. Such thermoplastic syntheticresins include a copolymer consisting of ethylene and vinyl monomercontaining carboxylate groups such as polyvinyl acetate, ethylene-vinylacetate copolymer, acrylic grafted polyethylene and ionomer, analiphatic polyamide such as polyhexamethylene adipamide and the like.

Multi-layered moldings obtained according to the present invention maybe used as a preform in a subsequent step such as blow molding, vaccumforming or forming under vacuum and pressurized air.

According to the present invention, a multiple layered molded articlemay be produced by a process comprising the steps of injecting first afirst kind of molten synthetic resin for forming the surface layers intoa mold cavity and subsequently injecting a second kind of syntheticresin as well as another body of said first kind of synthetic resin intosaid mold cavity in a concentric layered configuration, said injectionsteps being effected holding the volume of said first kind of syntheticresin firstly injected, the ratio of flow rate in the first to secondsynthetic resin simultaneously injected and the ratio of melt viscosityof the same within specific limitations.

Such specific limitations means that the volume of the first syntheticresin firstly injected is equal to that of the hardening layers formedat that portion of the first resin contacting the inner wall of the moldcavity immediately after the mold cavity is completely filled with thesynthetic resins, the melt viscosity of the second synthetic resin issmaller than that of the first synthetic resin and the melt viscositiesand shot volumes of the first and second resins are maintained such thatthese synthetic resins will flow translationally in the mold cavity in alaminar configuration without one covering the other at their leadingportions.

The specific limitations are concretely defined such that

M=L², Vi=2(Vm/lm) l_(o),

1<Q≦140, preferably 1<Q≦50, the most preferably 1<Q≦15,

1<M≦140, preferably 1<M≦50, the most preferably 1<M≦15, and

1<L<12

where Vi is the volume of the first synthetic resin firstly injected perunit width of the mold cavity in a direction perpendicular to the flowof resin therein, Q is the ratio of flow rate of the first to the secondsynthetic resin simultaneously injected, M is the ratio of meltviscosity of the same, L is the ratio of thickness of the firstsynthetic resin layer to the second synthetic resin layer, Vm is thevolume of the mold cavity per unit width thereof in a directionperpendicular to the flow of resin therein, lm is the thickness of thesame, and l_(o) is the thickness of the hardening layer formed adjacentto the inner wall of the mold cavity immediately after it is completelyfilled.

The multi-layered molded articles according to the present invention maybe produced under another specific limitation defined such that thevolume of the firstly injected synthetic resin is less than that of thehardening layers and the melt viscosities and shot volume of thesimultaneously injected synthetic resins are maintained in such a mannerthat these resins will flow in a laminar configuration in the moldcavity with the leading portion of the second resin flow being coveredby the first resin flow.

The limitation may be concretely defined such that

M<L² ##EQU1## 1<Q≦140, preferably 1<Q≦50, the most preferably 1<Q≦15,

1<M≦140, preferably 1<M≦50, the most preferably 1<M≦15, and

1<L≦80, preferably 1<L<12.

The present invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 illustrates the action of synthetic resin flowing within a flowpassage; (a) being from the direction parallel to the flow of resin, and(b) from fixing the viewpoint at the leading end of the flow

FIGS. 2 through 10 illustrate the action and state of the respectivesynthetic resin materials injected into a mold cavity, FIGS. 2, 3, 6 and7 illustrating the condition of the material injected according toprocesses other than the present invention, FIGS. 4 and 8 illustratingthe condition of the material injected according to the presentinvention, FIGS. 5 and 9 illustrating schematically various conditionsof flow at the leading portion in the mold cavity and FIG. 10illustrating the condition of synthetic resins injected according toanother embodiment of the present invention;

FIG. 11 is the graph showing a relationship between L, Q and M obtainedfrom the well known theory, the range of the present invention beingbounded by straight lines A, B, C and D;

FIG. 12 is a cross-sectional view showing the main portion in aninjection molding machine for carrying out the present invention;

FIG. 13 is a view similar to FIG. 12 showing another injection moldingmachine which is a second embodiment of the present invention;

FIG. 14 is a cross-sectional view showing the main portion of stillanother injection molding machine according to the present invention;

FIG. 15 illustrates a valve which may be located in the orifice of theinjecting cylinder as shown in FIG. 14; and

FIG. 16 is a graph showing the relationship between the melt viscosityand temperature of SAN resin.

The action of a synthetic resin injected into a mold cavity is generallycomplicated depending on the shape of the mold cavity and the like. Itis known in the art that this is illustrated as shown in FIG. 1a whenviewed in section in a direction parallel to the main flow of thesynthetic resin. In FIG. 1a, the injected synthetic resin flows withinthe mold cavity in a laminar configuration with the leading end facethereof being convex as shown by a solid line. The synthetic resin movestoward the inner walls of the mold cavity at the convex end face thereofas shown by dotted lines. When the resin is in contact with the innerwalls of the mold cavity, it results in hardening layers at both sidesof flow so that the central flow between said sides advances furtherforward to form sequentially convex leading end faces until the moldcavity is completely filled with the resin. As is illustrated moreclearly in FIG. 1b, therefore, the hardening layers are sequentiallyformed of the material supplied from the central flow. If a first kindof synthetic resin for forming surface layers is followed by a secondkind of synthetic resin for forming a core layer with the first resinbeing less than the second resin, the hardening layers would be formedof the second kind of synthetic resin in mid course.

The thickness of the hardening layer depends on the temperatures of theresin and mold, velocity of flow and the like. If these factors areappropriately selected, the thickness of the hardening layer can bedetermined as desired.

It is understood from the foregoing that a multi-layered article havingsurface layers and a core layer interposed therebetween can be producedif the material for forming the surface layers is equal in volume to thehardening layers to be formed.

FIG. 2 shows the prior art in which a first kind of resin 1 is firstinjected into a mold cavity in a volume sufficient to form hardeninglayers 2 and followed by a second kind of resin 3. During flowing withinthe mold cavity, as shown in FIG. 2a, the first resin 1 is present inadvance of the second resin 3. This body of the first resin 1 is forcedforward by the second resin continuously injected into the mold cavityto form sequentially the hardening layers 2 until an article is formedwith thinner hardening layers 5 adjacent to the inner wall of the moldcavity and a thicker core layer 6 surrounded by the hardening layers 5as shown in FIG. 2b.

FIG. 3 shows a state of flow in a process other than the presentinvention in which the first resin 1 is first injected into the moldcavity in a volume sufficient to form the hardening layers 2 andsubsequently another body of the first resin 4 is injected into the moldcavity together with the second resin 3 with the first resin positionedin advance of the leading portion of the second resin. During flowing,as shown in FIG. 3a, the first resin 4 is moved forward covering theleading portion of the second resin 3 and still increased progressivelyin amount. When the mold cavity is completely filled with the resins,therefore, the closed ends of the mold cavity are occupied by layers 5and 7 of the first resin with no second resin layer 6 as shown in FIG.3b.

FIG. 4 shows a state of flow in a process corresponding to a firstembodiment of the present invention in which the first resin 1 is firstinjected into the mold cavity in a volume equal to form the hardeninglayers 2 and followed by the second resin 3 as well as another body ofthe first resin 4, the first and second resin 4 and 3 being movedtranslationally without covering one another and flowing in a laminarform between the hardening layers 2 formed of the first resin 1 as shownin FIG. 4a. As seen from FIG. 4b, the second resin reaches the closedends of the mold cavity when it is completely filled.

FIG. 5 shows in enlarged scale the state of flow illustrated in FIG. 4.The first and second resins are in the conditions as shown in FIGS. 5aand 5b, respectively. FIG. 5a shows the second resin of relatively largeamount while FIG. 5b shows the second resin of relatively little amount.FIGS. 5a' and 5b' show the condition of flow corresponding to that ofFIGS. 5a and 5b after the time passed, respectively.

It is apparent from FIG. 5 that the second resin for the core layer mustbe moved translationally in a thin layered form so as to form the corelayer having its uniformly small thickness. This may be accomplished bymaintaining the melt viscosities and flow rates of the first and secondresins at a specific relationship therebetween according to the presentinvention.

In order to form a molded article having its uniform core layer of athickness smaller than one half of the entire wall thickness of thearticle, the second resin must have a melt viscosity smaller than thatof the first resin for forming the surface layers. If the second resinhas its melt viscosity equal to or greater than that of the first resin,the above article could not be made since the resin of greater viscositypresses the resin of smaller viscosity to form the second resin layerhaving the thickness equal to or greater than that of the first resinlayer when both of the first and second resins are injected at the sameflow rate into the mold cavity. If the second resin is decreased in flowrate relative to the first resin in order to avoid the above problem,the desired article could not be similarly produced since the secondresin will be covered by the first resin. If a second resin has a meltviscosity smaller than that of the first resin, the desired articlecould be made since the first resin presses the second resin to decreasethe thickness of the second resin layer whereby the linear velocity ofthe leading portion of the second resin becomes higher than that of thefirst resin so that the first and second resins will flowtranslationally without covering one another by selecting suitably theflow rate.

This can be quantitatively explained in another way by using the symbolswhich have been previously defined. Namely, Vi must be equal to 2·l_(o)·Vm/lm which indicates the volume of hardening layers per unit width ofthe mold cavity in a direction perpendicular to the flow of resintherein since the volume of first resin firstly injected should be equalto the above volume of hardening layers. Moreover, M must be equal to L²since the first and second resins should be translationally moved in themold cavity.

As described previously, the core layer could be formed of the resinsecondly injected into the mold cavity if the firstly injected resin hasits volume equal to that of the hardening layers to be formed.

If the first resin firstly injected has too much volume, the closed endof the mold cavity would be occupied by the firstly injected resin sothat the second resin secondly injected cannot reach the closed end ofthe mold cavity. If the first resin firstly injected has too littlevolume, the hardening layers would be formed of the second resin inpart. However, if the melt viscosities and shot volume of the first andsecond resins simultaneously injected in the second shot are determinedsuch that the first resin flows covering the leading portion of thesecond resin, the resin for forming the hardening layers would besupplied from the first resin secondly injected after the first resinfirstly injected converts completely into the portion of the hardeninglayers. It is understood, therefore, that the total volume of the firstresin firstly and secondly injected should be equal to the volumerequired to form the complete hardening layers in order to obtain thesecond resin layer formed substantially over the length of the moldcavity.

FIG. 6 shows the state of flow in a process other than the presentinvention in which the first resin firstly injected 1 has its volumemore than that of hardening layers to be formed. FIG. 6b shows thecondition under which a predetermined amount of the first and secondresins are completely charged into the mold cavity. It is understoodfrom FIG. 6b that the forward portion of the mold cavity adjacent to theclosed end thereof is occupied by the first resin firstly injected sothat the second resin layer does not reach the closed end of the moldcavity.

FIG. 7 shows the condition of flow in a process other than the presentinvention in which the first resin firstly injected has its volume lessthan that of the hardening layers to be formed. As seen from FIG. 7b,the first resin firstly injected is insufficient to form the hardeninglayers surrounding the entire core layer so that it is exposed outsidethe molded article.

FIG. 8 shows the state of flow in a process corresponding to a secondembodiment of the present invention in which the second resin layer as acore layer is fully covered by the first resin layers and formed up toadjacent the closed ends of the mold cavity.

FIG. 9 shows in an enlarged scale the state of the resin flow at itsleading portion. FIG. 9a shows the state of flow at a time immediatelyafter the first and second resins are simultaneously injected into themold cavity with the first resin used in relatively little amount. FIG.9b shows another state of flow at the same time as in FIG. 9a with thefirst resin used in relatively large amount. As seen from FIG. 9a', thesecond resin layer can reach the leading portion of the resin flow inthe mold cavity when the second resin is suitably covered by the firstresin. As seen from FIG. 9b', however, the second resin layer cannotreach the leading portion of the resin flow when the first resincovering the second resin is in too large an amount.

In the second embodiment of the present invention, the flow rate of thefirst resin is larger than that of the second resin due to the reasonwhy the first resin should be moved covering the leading portion of thesecond resin and also the thickness of the second resin layer should besmaller than that of the first resin layers. Therefore, the secondembodiment of the present invention is different from the firstembodiment thereof in that the desired articles can be produced bymaintaining the following limitations even if the melt viscosity of thesecond resin is not smaller than that of the first resin. The firstlimitation is such that the total volume of the firstly injected firstresin and sequentially injected first resin which covered the leadingportion of the second resin becomes equal to the volume of the hardeninglayers to be formed when the melt viscosity of the second resin is equalto that of the first resin. The other limitation is such that the flowrate of the first resin will be increased to prevent the second resinfrom pushing away the first resin when the melt viscosity of the secondresin is larger than that of the first resin. In said other limitation,the total volume of the firstly injected first resin and sequentiallyinjected first resin which covered the leading portion of the secondresin should similarly be equal to the volume of hardening layers to beformed. Thus, all of the molded articles produced according to thepresent invention will have the core layers of thickness substantiallyequal to one half of the entire wall thickness thereof.

On the contrary, when the melt viscosity of the second resin is smallerthan that of the first resin, the first resin presses the second resinto decrease the thickness thereof even if the flow rate of the secondresin is decreased substantially smaller than that of the first resin.This results in the linear velocity of the second resin layer beinghigher than that of the first resin layers so that the leading portionof the second resin layer will be excessively covered by the first resinlayers. Consequently, the second resin layer will reach adjacent to theclosed ends of the mold cavity in the form of a uniformly thin layer.

It is understood from the foregoing that the second embodiment of thepresent invention may provide the desired molded articles even if themelt viscosity of the second resin is not smaller than that of the firstresin as in the first embodiment thereof. Particularly, the secondembodiment of the present invention is preferred when the melt viscosityof the second resin is smaller than that of the first resin.

When the total volume of the firstly injected first resin andsequentially injected first resin which covered the leading portion ofthe second resin is smaller than the volume of the hardening layers tobe formed, the second resin may form a portion of the hardening layersat a position adjacent to the second resin layer as shown in FIG. 10.This is, of course, within the scope of the present invention. Thefirstly injected volume of the first resin can be optionally determinedwithin such range that it is sufficient to permit the second resin to beexposed outside at the initial injection and insufficient to prevent thesecond resin from reaching adjacent the closed ends of the mold cavity.

When the above limitation is explained by using the symbols which havebeen defined previously, it can be quantitatively represented asfollows:

Vi+V' must be smaller than 2·l_(o) ·Vm/lm which is the volume of thehardening layers since the total volume of the first resin which isfirstly injected and which is secondly injected together with the secondresin to be present in advance of the leading portion of the secondresin flow should be smaller than the volume of the hardening layers.The Vi+V' is necessarily larger than zero. The new symbol V' describedherein is the volume of the first resin which presents adjacent to theleading portion of the second resin flow simultaneously injectedtherewith.

Thus, a known theory will induce

    [Q/(Q+1)-6L.sup.2 (L+1)/{(M+L)+3L(L+1)}](Vm-Vi)

resulting is

    Vi <[1-{(M+L)+3L(L+1)}(Q+1)/{(M+L)+3L(L+1)}+6L.sup.2 (L+1)]Vm+[{(M+L)+3L(L+1)}(Q+1)/{(M+L)+3L(L+1)}+6L.sup.2 (L+1)]×2·Vm·lo/lm.

Furthermore, M must be smaller than L² due to the fact that the firstresin should flow covering the second resin layer.

FIG. 11 shows the relationship between the values of L, Q and M which isinduced based on the "Double-laminar flow" theory with respect toNewtonian fluid. Therefore, these values may be easily determined fromthe graph in FIG. 11. Practically, the respective values of L, Q and Mare limited within the following range.

For the purpose of the present invention, the thickness of the corelayer is desirably larger than or equal to 0.05 mm even if it is formedof any barrier resin which may be expected to perform the advantagethereof even in the form of an extremely thin layer. However, thethickness of the core layer may be smaller than 2 mm since any injectionmolded article does not normally have the wall thickness larger than 4mm. Consequently, the core layer according to the present invention ispreferably in the range of 0.05 to 2 mm in thickness except the upperlimit so that the value of L will be preferably in the range of 1 to 80except the lower limit. In order to satisfy the condition L² =M in thefirst embodiment of the present invention, the largest value of M willbe 80². This is impossible in practice. Taking into consideration thepossible range of M, the value L is particularly preferred to be in therange of 1 to 12 except the lower limit.

The value of Q is related to the value of M so that under the practicallimitation the both values of Q and M are in the range of 1 to 140except the lower limit, preferably 1 to 50 except the lower limit. Themost preferred range is from 1 to 15 except the lower limit. This is forthe reason described hereinafter.

When the ratio of flow rates is in the range of 1 to 15 except the lowerlimit, the present invention may be easily carried out by modifying thenozzle portion of any injection molding machine. When the ratio isbeyond 15, a flow-controlling device will be required to maintain thedesired conditions. When the ratio is beyond 50, the flow-controllingdevice will be more expensive.

It is to be noted that various limitations in numerals described hereinare based on round numbers, unless otherwise indicated specifically.

Upon attaining the process according to the present invention, thethickness of the hardening layers must be determined practically inorder to determine the volume of the first resin to be firstly injected.The thickness of the hardening layers depends on the temperature of theresin, the temperature of the mold, the shot velocity and the like. Whensuch conditions are maintained constant, the thickness can be readilydetermined from some experiments attained under these conditions.

The experiments are accomplished in such a manner that the first resinis first of all injected into the mold cavity and subsequently only thesecond resin is injected thereinto with no first resin to form a moldedarticle having the second resin layer substantially over the lengththereof. The resultant article is measured at various surface portionsthereof to obtain the average thickness of the first resin layer. Eitherof the first or second resin is preferably colored for easy measuringoperations.

FIG. 12 shows an injection molding machine for attaining the processaccording to the present invention in which an injecting cylinder 10 hasan accumulating bore 11. Within the bore 11 there is mounted slidably aplastifying screw 12 which may be used as an injecting ram.

A projecting portion 13 extends rearward from the forward end of theaccumulating bore 11 to define a substantailly annular space 14. Theouter periphery 15 of the projecting portion 13 supports slidably amovable mandrel 16 which is mounted slidably within the bore 11 of thecylinder 10. The mandrel 16 has a central passage 17 passedlongitudinally therethrough which is fitted over the outer periphery 15of the projecting portion 13 so that the annular space 14 will be closedby the movable mandrel 17.

The annular space 14 is connected with a plastifying cylinder 18 forsupplying a second resin to be formed into a core layer. A first resinfor forming surface layers is supplied to the accumulating bore 11 bymeans of the plastifying screw 12.

The projecting portion 13 has a central passageway 19 which is connectedthrough a channel 20 to a nozzle portion described hereinafter. Theannular space 14 around the projecting portion 13 also is connectedthrough another channel 21 to the nozzle portion. These channels 20 and21 are controlled by means of a rod-like member or cock 22 actuated byany suitable means such as an actuator 23. The rod-like member includesports 24 and 25 formed therein which are disposed perpendicularly to thelongitudinal axis of the member 22.

The injecting cylinder 10 is provided with a nozzle portion 26 includinga nozzle space 27 which communicates with an injection port 28 formed inthe most forward end of the injecting cylinder 10. The nozzle space 27has at the forward portion thereof a substantially conical inner wall 29formed adjacent to the injection port 28.

A nozzle member 30 is located within the nozzle space 27 which has asubstantially cylindrical body 31 and a substantially conical head 32.The cylindrical body 31 has a passage 33 communicating with the channel21 of the cylinder 10. The passage 33 also is connected to the forwardportion of the nozzle space 27 through an orifice 34 formed in theconical head 32 of the nozzle member 30. The conical head 32 is spacedaway from the conical wall 29 of the nozzle space 27 to form aflow-passage 35 communicating with the forward portion of the nozzlespace 27 and thus the injection port 28 of the nozzle portion 26.

A rectifying ring member 36 is located spaced away from and around theouter periphery of the cylindrical body 31. The rectifying ring member36 has a plurality of openings through which the flow-passage 35 isconnected to the channel 20 of the cylinder 10.

A nozzle ring 40 is located adjacent to the conical head 32 of thenozzle member 30 around the flow-passage 35. The nozzle ring 40 isradially moved by adjusting screws 41 to bring it into extension ofretraction from the conical wall 29 of the nozzle space 27.Consequently, the flow-passage 35 can be controlled by the nozzle ring40 to change the flow balance of the first resin which has flowed fromthe accumulating bore 11 through the passageway 19, the channel 20 andthe rectifying ring member 36 to the flow-passage 35.

Supposing that the first and second resins are accumulated within theaccumulating bore 11 and annular space 14 by means of the plastifyingscrew and cylinder 12, 18, respectively, the cock 22 is positioned insuch a position that the port 24 is opened and the port 25 is closed asshown in FIG. 12 prior to the injection operation. When the injectingscrew 12 is moved forward, only the first resin is injected into a moldcavity (not shown) through the central passage 17 of the mandrel 16, thepassageway 19 of the projecting portion 13, the channel 20, the port 24of the cock 22, the flow-passage 35 and the injection port 28.

When a predetermined amount of the first resin has been injected intothe mold cavity, the cock 22 is rotated by means of the actuator 23 toposition the ports 24 and 25 thereof in such a position that apredetermined ratio of flow rate in the first and second resins isattained. Thereafter, a second shot is effected by the injecting screw12 through the mandred 17. In this time, the second resin is injectedfrom the annular space 14 through the channel 21, the port 25, thepassage 33, the orifice 34 and the injection port 28 to the mold cavityby the forward movement of the mandrel 16.

The ratio of flow rate may be determined optionally by suitablyselecting the diameter and angular relationship between the ports 24 and25 of the cock or rod-like member 22.

Referring to FIG. 13, the machine comprises an injecting cylinder 50having a injecting screw 51 which is received slidably within theaccumulating bore 52 of the cylinder 50. This injecting screw 51 issimilar to the screw 12 in FIG. 12.

Similarly, a movable mandrel 53 is slidably mounted within the bore 52which has at its forward end an extension 54. The mandrel 53 is providedwith a central passage 55 passed longitudinally throughout the body andextension 54 thereof.

The injecting cylinder 50 has at its forward portion a projectingportion 56 extending rearward within the accumulating bore 52 thereof.The projecting portion 56 has a passageway 57 formed therein along thecentral longitudinal axis of the portion 56. This passageway 57 isadapted to receive the extension 54 of the mandrel retaining a spaceinterval between the outer periphery of the extension 54 and the innerperiphery of the passageway 57.

The inner wall of the bore 52, the forward end of the movable mandrel 53and the extension 54 thereof define a substantially annular space 59which is connected to a plastifying cylinder 60 for supplying the firstresin to be formed into surface layers. This cylinder 60 is similar tothe plastifying cylinder in FIG. 12. The second resin for forming a corelayer is similarly supplied to the bore 52 behind the mandrel 53 bymeans of the injecting screw 51.

Tubular passages 58 are formed around the projecting portion 56. Thetubular passages 58 are controlled by adjusting means 61 to change theflow rate of the first resin flowing therethrough. The projectingportion is included in a nozzle portion 62 which is located at theforward portion of the injecting cylinder 50. The annular space 59 isconnected through the tubular passages 58 and annular orifice 63 to thesubstantially conical nozzle space 64 of said nozzle portion 62. Thenozzle space 64 is connected coaxially to an injection port 65 formed inthe most forward end of the injecting cylinder 50. The passageway 57 ofthe projecting portion 56 is disposed coaxially with the injection port65.

The annular orifice 63 can be controlled by means of a nozzle ring 66and adjusting screws 67 similar to the nozzle ring and screws in FIG.12.

Upon accumulating the first and second resins within the respectivespace 59 and bore 52, an initial injecting operation is attained by theforward movement of the injecting cylinder screw 51. The first resin tobe firstly injected is determined by the distance between the forwardend of the extension 54 and the corresponding rearward end of theprojecting portion 56. Namely, when the extension 54 is spaced away fromthe rearward end of the projecting portion 56, the first resin only isinjected through the passageway 57 of the projecting portion 56, thenozzle space 64 and the injection port 65 to a mold cavity (not shown)since the flow resistance in the passageway 57 is less than that in thecentral passage 55 of the mandrel 53. However, the first resin will beinjected through the passageway 57, the nozzle space 64 and theinjection port 65 immediately before or when the extension 54 of themandrel 53 is received in the passageway 57 of the projecting portion56. In this time, if the tubular passages 58 is decreased incross-section by the adjusting means 61 to equalize the flow resistancein the tubular passages 58 to that in the central passage 55 of themandrel 53, the second resin will be simultaneously injected through thecentral passage 55 from that portion of the bore 52 behind the mandrel53.

FIG. 14 shows an injection molding machine which is still anotherembodiment of the present invention. The machine comprises a firstinjecting cylinder 70 having a first accumulating chamber 71 withinwhich a first injecting ram or screw 72 for plastifying and injectingthe resin material is housed.

A nozzle portion 73 is formed integrally in the forward end of the firstinjecting cylinder 70. The nozzle portion 73 defines substantiallycylindrical space 74 which communicates with the first accumulatingchamber 71 through a reduced bore 75 therebetween. The forward end ofthe nozzle portion 73 is provided with a injection port 76 which isadapted to connect with a mold cavity (not shown).

Within the cylindrical space 74 of the nozzle portion 73 there islocated a second injecting cylinder 77 the forward end of which isspaced away from the inner wall of the nozzle portion except its base 78to form a substantially annular passage 79 for the resin materialflowing from the first accumulating chamber 71 of the injecting cylinder70. The second injecting cylinder 77 has at its forward and rearwardends outer substantially conical wall portions which definesubstantially conical channels 80 and 81 together with the inner conicalwalls of the nozzle portion 73. The channels 80 and 81 communicate withthe injection port 76 and reduced bore 75, respectively.

A nozzle ring 82 is located adjacent to the injection port 76 around theforward conical channel 80 for controlling this channel by actuatingscrew means 83 in a manner similar to that in FIG. 12.

The second injecting cylinder 77 has a second accumulating chamber 84formed therein coaxially to the first accumulating chamber 71 of thefirst cylinder 70. The second accumulating chamber 84 is connected tothe injection port 76 through an orifice 86 formed in the forward end ofthe second cylinder 77 coaxially to the injection port 76. The secondaccumulating chamber 84 also is connected through a duct 88 to anextruder 89 which supplies the resin material to the chamber 84 througha check valve 90.

The second accumulating chamber 84 has at its rearward end an openingwithin which a second injecting ram 91 is slidably received. The secondram 91 has at its rearward end a recess 92 for engaging the forward endof the first injecting ram 72.

In such an arrangement, the first resin for the surface layers isplastified and accumulated in the first chamber 71 by means of the firstscrew 72 while the second resin for the core layer is plastified andaccumulated in the second chamber 84 by means of the extruder 89. When apredetermined amount of the resin material is charged into therespective chambers, the first screw 72 is moved forward to inject firstof all the first resin in the first chamber 71 through the channels 81,79 and 80 and the injection port 76 into the mold cavity. Furtherforward movement of the first ram 72 causes it to engage the second ram91 so that it moves forward in the second chamber 84 to inject thesecond resin out of the second chamber 84 through the orifice 86. Thus,the second injected resin is injected through the injection port 76together with the first resin from the channel 80 in the form ofconcentric circles.

The dimensions of the orifice and port 86, 76 should be appropriatelydetermined such that the second resin is not simultaneously injected asthe first resin is firstly injected. Any suitable valve means as shownin FIG. 15 may be located in the orifice 86 of the second cylinder 77 ifdesired. In FIG. 15, 101 is a needle valve and 102 is a backup spring.

Once the second ram 91 has been engaged by the first ram 72, the ratioof shot volume in the first resin to the second resin depends on thedifference between the inner diameters of the cylinders 70 and 73. Ifthe inner diameter of the second cylinder 77 is suitably selected,therefore, the desired ratio of shot volume can be obtained. Anysuitable means for changing the inner diameter of the second cylinder 77may be provided.

Examples of the process according to the present invention will now bedescribed. Conditions of molding in these examples are indicated byTable 1 in which the examples indicated by Nos. 1 to 8 were attained byusing the injection molding machine shown in FIG. 12 to form cylindricalclosed-end parisons, each having an outer diameter of 4 cm, length of 20cm and thickness of 4 mm.

The values M in Table 1 were obtained by the graph shown in FIG. 16. TheSAN of barrier and good flow properties had melt viscosities of about3×10² poise and about 60 poise at the resin temperature of 260° C.,respectively. The GPPS had a melt viscosity of about 3×10³ poise at 180°C. and the HIPS had a melt viscosity of about 2×10³ poise at 190° C.These values of poise were all measured at a shear rate of about 10⁴sec⁻¹.

In Table 1 as well as Table 2, "SAN" was a copolymer resin consisting of30% by weight acrylonitrile and 70% by weight of styrene, "barrier SAN"was a copolymer resin consisting of 75% by weight acrylonitrile and 25%by weight styrene, "GPPS" was a polystyrene resin and "HIPS" was ahigh-impact polystyrene resin.

The examples indicated by Nos. 9 and 10 were attained by using theinjection molding machine shown in FIG. 14 to form cylindricalclosed-end parisons each having an outer diameter of 4 cm, length of 20cm and thickness of 4 mm.

In examples of Nos. 9 and 10, "PE" was a low density polyethylene ofmelt index 7 g/10 min. "Blend A" consisted of 50% by weight saponifiedethylene-vinyl acetate copolymer containing 25 mol % ethylene and 50% byweight low density polyethylene. "Blend B" consisted of 50% by weightsaponified ethylene-vinyl acetate copolymer containing 25 mol % ethyleneand 50% by weight low density polyethylene and further containingethylene-vinyl acetate in an amount of 10 parts by weight per 100 partsby weight of the blend in which carbonyl group were present at aconcentration of 450 meq/100 g. At the respective temperatures shown inTable 1, the melt viscosity of PE was about 3×10³ poise, Blend A wasabout 2×10³ poise and Blend B was about 2×10³ poise.

Molded articles obtained in the examples of Table 1 are indicated at thenumbers of Table 3 corresponding to the numbers of Table 1.

No. 8 article in Table 3 was injection molded to have the core layer ofHIPS formed adjacent to the outer surface thereof. Upon applying impact,this article was cracked but not broken. No. 11 article in Table 3 wasinjection molded of the first transparent resin and the second coloredresin under the same conditions as in No. 3 of Table 1. The resultantarticle was of superior appearance with the colored core layer coveredby the thick transparent surface layers. Nos. 1 to 7 articles haduniform core layers extending over the length thereof adjacent to theinner surfaces of the articles except that the core layer of No. 7article was not uniform only at the extremity thereof.

Nos. 9 and 10 articles had the core layers extending over the lengththereof and could not be separated at the connection between the firstand second resin layers even by applying strong impact. The gas-barrierproperties thereof also were superior.

Comparison examples were attained under conditions shown in Table 2 inwhich the resultant articles are indicated in Table 4 by thecorresponding numbers thereof. The articles of Nos. 1 and 2 all had thecore layers not extending over the length thereof while the articles ofNos. 3 and 4 had the core layers exposed at the surfaces thereof.

                  Table 1                                                         ______________________________________                                        (Examples)                                                                                           Tem-        First                                                             pera-       resin                                                             ture Volume firstly                                                 Kind      of   of     in-                                                     of        resin                                                                              resin  jected                                     No.          resin     (°C.)                                                                       (cc)   (cc)  L   Q   M                            ______________________________________                                        1    First   SAN       185  102    48    3   3   9                                 resin                                                                         Second  Barrier                                                               resin   SAN       260  18     --                                         2    First   SAN       195  102    30    3   4   6                                 resin                                                                         Second  Barrier                                                               resin   SAN       260  18     --                                         3    First   SAN       180  78     24    1.3 1.3 1.7                               resin                                                                         Second  SAN       195  42     --                                              resin                                                                    4    First   SAN       180  75     5     1.1 1.6 1.1                               resin                                                                         Second  SAN       185  45     --                                              resin                                                                    5    First   SAN       180  90     30    2   2   4                                 resin                                                                         Second  SAN       225  30     --                                              resin                                                                    6    First   SAN       180  96     15    2.3 3.4 1.5                               resin                                                                         Second  SAN       190  24     --                                              resin                                                                    7    First   SAN       180  117    3     31  38  50                                resin                                                                         Second  Good      260  3      --                                              resin   flow                                                                          proper-                                                                       ties                                                                          SAN                                                              8    First   GPPS      180  96     15    2.3 3.4 1.5                               resin                                                                         Second  HIPS      190  24     --                                              resin                                                                    9    First   PE        180  96     15    2.3 3.4 1.5                               resin                                                                         Second  Blend A   190  24     --                                              resin                                                                    10   First   PE        180  96     15    2.3 3.4 1.5                               resin                                                                         Second  Blend B   180  24     --                                              resin                                                                    ______________________________________                                    

                  Table 2                                                         ______________________________________                                        (Comparison Examples)                                                                                            First                                                           Tempera-                                                                             Volume resin                                                    Kind   ture of                                                                              of     firstly                                                  of     resin  resin  injected                                   No.           resin  (°C.)                                                                         (cc)   (cc)    Q   M                              ______________________________________                                        1    First    SAN    260      102  48      3   0.1                                 resin                                                                         Second   SAN    180      18   --                                              resin                                                                    2    First    SAN    260      102  30      4   0.2                                 resin                                                                         Second   SAN    180      18   --                                              resin                                                                    3    First    SAN    180      70    7      1.3 1.5                                 resin                                                                         Second   SAN    190      50   --                                              resin                                                                    4    First    SAN    180      55    7      0.7 10                                  resin                                                                         Second   SAN    260      65   --                                              resin                                                                    ______________________________________                                    

                  Table 3                                                         ______________________________________                                        (Examples)                                                                                 Thick-                                                                Thick-  ness of                                                               ness    har-                                                                  of core dening                                                                layer   layer                                                            No.  (mm)    (mm)      State of core layer                                    ______________________________________                                        1    0.6     0.8     Uniform core layer extending                                                  over the length of the                                                        article adjacent to the                                                       inner face thereof                                       2    0.6     0.8     "                                                        3    1.4     0.4     "                                                        4    1.5     0.4     "                                                        5    1.0     0.5     "-6               0.8 0.7 "                              7    0.1     0.8     "                                                        8    0.8     0.7     Uniform core layer extending                                                  over the length of the                                                        article adjacent to the                                                       outer face thereof                                       9    0.8     0.7     Uniform core layer extending                                                  over the length of the                                                        article adjacent to the                                                       inner face thereof                                       10   0.8     0.7     "                                                        11   1.4     0.4     "                                                        ______________________________________                                    

                  Table 4                                                         ______________________________________                                        (Comparison) Examples)                                                              Thickness  Thickness                                                          of core    of harden-                                                         layer      ing layer                                                    No.   (mm)       (mm)       State of core layer                               ______________________________________                                        1     1.2        0.8        Core layer extending over                                                     50% of the length of the                                                      article                                           2     1.2        0.6        "                                                 3     --         --         Core layer exposed at the                                                     mid portion of the article                        4     --         --         "                                                 ______________________________________                                    

We claim:
 1. A multi-layered molded article having the shape of avessel, closed-end parison, disk or the like, said article comprising inits cross-section a triple-layered structure consisting of two surfacelayers of different thicknesses and a core layer interposed between saidsurface layers, said triple-layered structure having been injectionmolded in a mold cavity in a single shot from two different kinds ofthermoplastic synthetic resin to form said surface and core layers,respectively, said core layer having a thickness less than one half ofthe entire thickness of said article, said triple-layered structurehaving its crosssection represented by

    a+b<1/2(a+b+c)

where a is the thickness of the core layer, b is the thickness of thethinner surface layer and c is the thickness of the thicker surfacelayer.
 2. The multi-layered molded article as set forth in claim 1,wherein the value of (a+b+c) is less than or equal to 4 mm.
 3. Themulti-layered molded article as set forth in claim 1, wherein the valueof a is less than 1 mm but greater than or equal to 0.05 mm.
 4. Themulti-layered molded article as set forth in claim 1, said articlehaving been produced by first injection molding said synthetic resinsinto a cylindrical preform and subsequently blow molding said preform.5. The mult-layered molded article as set forth in claim 1, said articlehaving been produced by first of all injection molding said syntheticresins into a disk-like or sheet-shaped preform and subsequently formingsaid preform under vacuum or under vacuum and pressurized air.
 6. Themulti-layered molded article as set forth in claim 1, wherein thethermoplastic synthetic resin forming said core layer is a gas barrierresin.
 7. The multi-layered molded article as set forth in claim 6,wherein said gas barrier resin is at least one member selected from thegroup consisting of a vinyl chloride-vinylidene chloride copolymer,saponified ethylene-vinyl acetate copolymer, nylon, and aacrylonitrile-styrene copolymer containing a major amount ofacrylontrile.
 8. The multi-layered molded article as set forth in claim6, wherein said synthetic resin used to form said surface layers is apolyolefin, and said barrier resin is a blend containing a polyolefin aswell as at least a saponified ethylene-vinyl acetate copolymer, theweight ratio of said polyolefin and saponified ethylene-vinyl acetatecopolymer in said blend being 10-90:90-10.
 9. The multi-layered moldedarticle as set forth in claim 8, wherein said blend consists of asaponified ethylene-vinyl acetate copolymer, polyolefin andthermoplastic polymer in which carbonyl groups are present at aconcentration of 120-1400 meq per 100 g of said polymer at its backbonechain or side chain, said polymer being present in the range of 0.5 to15 parts by weight per 100 parts by weight of the total amount of saidpolyolefin and saponified ethylene-vinyl acetate copolymer.
 10. Themulti-layered molded article as set forth in claim 1, wherein thethermoplastic synthetic resin used to form said core layer hashigh-impact properties.
 11. The multi-layered molded article as setforth in claim 10, wherein said high-impact synthetic resin is selectedfrom the group consisting of a acrylonitrile-butadiene-styrenecopolymer, acrylonitrile-styrene copolymer, high-impact polystyrene,polyethylene terephthalate, nylon, polyacetal and polycarbonate.
 12. Themulti-layered molded article as set forth in claim 1 wherein the valueof (a+b+c) is less than or equal to 4 mm, the value of a is less than 1mm but greater than or equal to 0.05 mm, the thermoplastic syntheticresin forming said core layer is a gas barrier resin including at leastone member selected from the group consisting of a vinylchloride-vinylidene chloride copolymer, saponified ethylene-vinylacetate copolymer, nylon and an acrylonitrile-styrene copolymercontaining a major amount of acrylonitrile, and the synthetic resin usedto form the surface layers is a polyolefin.
 13. The multi-layered moldedarticle as set forth in claim 12 wherein said gas barrier resin is ablend containing a polyolefin as well as at least a saponifiedethylene-vinyl acetate copolymer, the weight ratio of said polyolefinand saponified ethylene-vinyl acetate copolymer in said blend being10-90:90-10.